1. Field of the Invention
The invention relates generally to superparamagnetic nanostructured materials and a method for making the same.
2. Background Information
Engineering synthetic materials towards the molecular architecture of biological molecules is a powerful concept. These synthetic materials may be efficiently integrated with biological molecules for further detection or targeting of biological tissues. Synthetic material that is fabricated near an atomic length scale xcx9c10 nm is particularly challenging since this is a lower limit on which important biological processes and mechanisms occur in nature. Of particular interest is the synthesis of magnetic particles approaching the 5 nm length scale. Magnetic particles can be used as markers for the detection of biological molecules or in a drug delivery system to target infected biological tissues. The detection and targeting may be monitored or controlled using the magnetic properties of the particles. In terms of medical needs, magnetic particles are used as directed delivery agents for radionuclides as a method of tumor treatment. Also, magnetic particles can be utilized in sensor and separation technology. For example, the surface of the magnetic particles could be functionalized with specific biomolecular recognition sites, cells or proteins to detect antibodies through separation of a fluidic medium using magnetic fields.
It is well known that superparamagnetic particles only exhibit magnetization in the presence of a magnetic field. A superparamagnetic nanostructure having a well-controlled shape, size, and composition would be useful in the biological field, as well as many other fields. Superparamagnetic nanostructures can be easily introduced into a biological fluid without the concerns of agglomeration due to ferromagnetic attraction that would be present in other non-superparamagnetic particles and nanostructures. Additionally, the presence and location of the superparamagnetic nanostructures can be detected using a magnetic field.
Iron silicates particles exhibit superparamagnetic properties below a critical size. Beginning in the early 1980""s, the characterization of iron silicates provided useful early information concerning iron species at different calcinations temperatures and a limited study of magnetic properties. In addition, these earlier studies developed a synthesis strategy for producing bulk iron silicates whereby metal alkoxides, specifically iron triethoxide and iron tripropoxide, were mixed with silicon precursors, most commonly iron salts were used such as iron halides (FeCl3) and organic salts like iron nitrate. Moving to the late 1990s, more extensive magnetic measurements were reported on bulk iron silicate composites. Several systems involving silica precursors and iron alkoxides or iron salts produced the phase, xcex3-Fe2O3 (maghemite), which in bulk form is ferromagnetic. The xcex3-Fe2O3 particles found in the silicates were below the critical size for ferromagnetism resulting, however, in superparamagnetic behavior of the composites. Superparamagnetic iron (III) nanoparticles were produced having a narrow size distribution, 4-6 nm, with varying degrees of iron salt content. It was determined that a transition to ferromagnetism occurs at lower temperatures and increased particle size. The transition was clarified to indicate that at low concentrations of Fe2O3 the change occurs at a particle size of 5 nm. Most studies included the effects of thermal treatments on silica composites to maximize magnetization. Two factors contributed to predicting the magnetic moment of an iron silicate, the concentration of iron to that of silica and the various temperature treatments. In general, research has indicated that maximizing iron content and administering calcinations temperatures in the range of 600-900xc2x0 C. lead to increasing amounts of maghemite, xcex3-Fe2O3, and yielded high magnetic moments. Heretofore, conventional techniques for making superparamagnetic particles have been limited to the production of spherical particles.
Diamagnetic aluminosilicate mesostructures have been synthesized from block copolymer phases. By increasing the fraction of inorganic precursors, 3-glycidyloxy-propltrimethoxysilane and aluminum-sec-butoxide, expected block copolymer morphologies were exhibiting phase separation on the length scale of about 20 nm. Later studies extended this approach to produce single nano-objects. The hydrophilic part of the block copolymer was embedded in the inorganic phase while the hydrophobic part forms the second phase. Then an organic solvent dissolves the bulk materials leading to xe2x80x9chairyxe2x80x9d objects. Upon heating at high temperatures (xcx9c600xc2x0 C.), aluminosilicate spheres, cylinders, and plates of controlled shape, size, and composition result. However, there have been no reports relating to the production of superparamagnetic nanostructured material having these various shapes. In particularly, there have been no reports relating to the production of mesoporous nanostructures exhibiting superparamagnetic. Mesoporous nanostructures would be viable structures in filtration processes and as a catalytic material.
Some success with synthesizing magnetic nano-objects on the basis of block copolymer structure direction has already been reported using a particular tri-block copolymer system. Through chemistry on block copolymer spheres in solution, iron was deposited in the cores of a selected tri-block copolymer while still maintaining the integrity of the spheres. The nanosphere size ranged from 4-16 nm and displayed qualitative magnetic properties. The reported construction and study of magnetic nanorods made from uniform magnetic nanosphere presents other interesting issues. A rod shaped magnetic nano-object introduces the effect of shape anisotropy. Shape anisotropy is present when magnetization is achieved more readily along certain axis. For a rod, the long axis is more easily magnetized than the short axis. The presence of shape anisotropy would lead to more individual nanorod moments being aligned, and thus rendering a larger overall moment. Therefore, superparamagnetic nanostructured materials that are not limited to spherical geometries would be advantageous.
Additionally, the use of amphiphilic block copolymers as structure-directing agents to generate nanostructured material is also known. However, prior attempts at generating nanostructured material do not appear to focus on the magnetisms of the nanostructured material produced. Particularly, the use of amphiphilic block copolymers to produce superparamagnetic nanostructured material has not been explored.
Superparamagnetic nanostructured materials are produced using an amphiphilic block copolymer having the form AB, ABA, or ABC as a structure directing agent. The method is unique in attaining unprecedented structural control over the formation and composition of superparamagnetic nanostructured materials. Whereas conventional techniques are limited to spherical particles, the present approach can be conveniently extended to cylindrical and lamellar shapes. Most importantly, the present approach may be used to produce mesoporous nanostructures exhibiting superparamagnetism. These mesoporous nanostructures are useful as filtration devices and catalytic material.
Superparamagnetic nanostructured materials are formed by preparing a block copolymer solution containing an amphiphilic block copolymer. The block copolymer should have the form AB, ABA, or ABC, such that one of the constituents is a hydrophilic polymer. In one preferred embodiment, the amphiphilic block copolymer solution is formed by dissolving poly(isoprene-block-polyethylene oxide), which may be denoted PI-b-PEO, in a non-aqueous solvent. A silicate precursor solution is then formed, preferably in an aqueous solution. The silicate precursor solution undergoes hydrolysis and condensation to form a sol-gel precursor. In one preferred embodiment, an aluminum-containing compound is mixed with the silicate precursor solution. An iron precursor is added to either the block copolymer solution or the sol-gel precursor. In a preferred embodiment, iron alkoxide is added to the block copolymer solution. The sol-gel precursor is mixed with the block copolymer solution whereby a hybrid inorganic nanostructured material is produced. Solvent is removed from the hybrid inorganic nanostructured material to produce individual nanostructured material which are calcinated to form a superparamagnetic nanostructure material. In a preferred embodiment, the superparamagnetic nanostructured material is superparamagnetic aluminosilicate nanostructured material. Although the resulting superparamagnetic nanostructure material is preferably in the shape of a sphere, cylinder or lamellae, the most preferred shape is a mesoporous material.